Magnetically soft powder composite material, method for manufacturing same, and its use

ABSTRACT

A magnetically soft powder composite material is described, which is composed of at least 99.4 wt. % of a pure iron powder, a phosphatized iron powder, or an iron alloy powder and 0.05 wt. % to 0.6 wt. % of a soft ferrite powder and which is primarily suited for use in rapidly switching solenoid valves in motor vehicle engines. Furthermore, a method for manufacturing such a magnetically soft powder composite material includes the following method steps:
     a) preparation of a starting mixture including a pure iron powder, a phosphatized iron powder, or an iron alloy powder and a soft ferrite powder,   b) mixing of the starting mixture,   c) compacting of the starting mixture in a press under increased pressure,   d) debinding of the compacted starting mixture in an inert gas atmosphere or in an oxygen-containing gas atmosphere, and   e) heat treatment of the compacted starting mixture in an oxidizing gas atmosphere at a temperature of 410° C. to 500° C.

FIELD OF INVENTION

The present invention relates to a magnetically soft powder compositematerial, and a method for manufacturing such a material.

BACKGROUND INFORMATION

Modern gasoline engines and diesel engines require increasinglyefficient solenoid injectors in order to meet the demands for reducingfuel consumption and pollutants, for example. Rapidly switching solenoidinjectors are manufactured using magnetically soft materials, such asFeCr alloys or FeCo alloys, or powder composite materials having anintrinsic electrical resistance as high as possible. However, due toalloy-associated measures, only an intrinsic electrical resistance of 1μΩm maximum is achievable in metallic materials.

Furthermore, a magnetic material composed of iron powder and an organicbonding agent may be used in valves for diesel injection (common railsystem). Although these materials have higher intrinsic electricalresistances than the aforementioned magnetically soft alloy materials,they are limited in many cases with respect to their fuel stability andthermal stability and are also poorly processable.

German Published Patent Application No. 199 60 095 describes a sinteredmagnetically soft composite material and a method for its manufacture inwhich a ferromagnetic starting component as the main component and aferritic starting component as a minor component are used in a startingmixture from which, after a heat treatment, a magnetically softcomposite material is formed. After the heat treatment of the startingmixture forming the composite material, the second starting componentrepresents a grain boundary phase. The first starting component is apure iron powder or a phosphatized iron powder, for example; the secondstarting component is a ferrite powder, e.g., a soft ferrite powder,such as MnZn ferrite or NiZn ferrite. The proportion of the iron powderin the starting mixture equals 95 percent to 99 percent by weight, andthe proportion of the ferrite powder equals 1 percent to 25 percent byweight.

SUMMARY

It is an object of the present invention to provide a magnetically softpowder composite material which has a magnetic saturation polarizationand magnetic permeability which are as high as possible combined with anintrinsic electrical resistance which is as high as possible.

The magnetically soft powder composite material may provide that it hasa magnetic saturation polarization of more than 1.85 Tesla, e.g., 1.90Tesla to 2.05 Tesla, and, that it has a clearly elevated intrinsicelectrical resistance of more than 1 μΩm, e.g., of 5 μΩm to 15 μΩm. Theintrinsic electrical resistance lies at approximately 10 μΩm. Inaddition, the magnetically soft powder composite material according tothe present invention may have a flexural strength of more than 120 mPa,measured from cylindrical samples. The edge fracture strength of thecomponents made of this material in the form of solenoid cups forinjectors is over 45 kN, and, in addition, the achieved magneticallysoft powder composite material is thermo-stable and fuel-stable at atemperature of up to at least 400° C. Therefore, the material is verywell suited for manufacturing rapidly switching solenoid valves of thetype required for diesel injection in motor vehicle engines.

The method according to the present invention for manufacturing themagnetically soft powder composite material provides for adding apressing support arrangement, a micro wax for example, to the startingmixture facilitates pressing and that the properties of the achievedpowder composite material may be easily adjusted via the gas atmosphereand the temperature program during debinding or during the heattreatment.

The utilized soft ferrite powder may be an MnZn ferrite powder, an NiZnferrite powder, or a mixture of both powders. The powder particles ofthe utilized pure iron powder, the iron alloy powder, or the utilizedphosphatized iron powder may have an average grain size of between 30 μmand 150 μm, while, in contrast, the grain size of the utilized softferrite powder is clearly smaller and averages less than 20 μm. Theaverage grain size of the utilized soft ferrite powder particles may beless than 5 μm, e.g., less than 1 μm.

DETAILED DESCRIPTION

The manufacture of the magnetically soft powder composite materialstarts with a starting mixture composed of a pure iron powder or aphosphatized iron powder and a soft ferrite powder. Iron alloy powders,such as FeCr powder or FeCo powder, may also be used as an alternativeto the iron powder.

Phosphatized iron powder may be used since it achieves the bestelectrical properties of the powder composite material.

Furthermore, a pressing support arrangement, such as a micro wax, mayalso be added to the starting mixture, the pressing support arrangementbeing removed again during the course of a subsequent heat treatment ofthe starting mixture for manufacturing the magnetically soft powdercomposite material. The proportion of the pressing support arrangementin the starting mixture is 0 wt. % to a maximum of 0.8 wt. %. Apart fromthe pressing support arrangement, the starting mixture is composed of atleast 99.4 wt. % of a pure iron powder or a phosphatized iron powder and0.1 wt. % to 0.6 wt. % of a soft ferrite powder. The proportion of thepure iron powder or the phosphatized iron powder may equal more than99.5 wt. %, e.g., 99.7 wt. % to 99.8 wt. %. The proportion of the softferrite powder may equal less than 0.5 wt. %, e.g., 0.1 wt. % to 0.3 wt.%. Unavoidable contaminations or negligible residues of the initiallyadded pressing support arrangement which are possibly still present havebeen neglected in this calculation of the composition of the achievedmagnetically soft composite material which materializes after themixing, compressing, debinding, and the heat treatment of the initiallycreated starting mixture.

The utilized soft ferrite powder may be a manganese-zinc ferrite(MnZnOFe₂O₃) or a nickel-zinc ferrite (NiZnOFe₂O₃), or a mixture of bothpowders. Phosphatized iron powder or phosphatized pure iron powder andone of these two soft ferrite powders may be used.

The powder particles of the pure iron powder or the phosphatized ironpowder have an average grain size of 50 μm to 100 μm. The grain size ofthe utilized soft ferrite powder may be distinctly below 20 μm, e.g.,below 5 μm. It is, for example, in the range between 0.5 μm and 2 μm,e.g., around 1

Moreover, it should be pointed out that, depending on the intendedapplication of the achieved material, during the composition of thestarting mixture, which is made up of the pure iron powder or thephosphatized iron powder and the soft ferrite powder, more importancemay be attached to a magnetic saturation polarization and magneticpermeability which are as high as possible, i.e., μ_(max) greater than800, or to an intrinsic electrical resistance which is as high aspossible by varying the composition of the material.

The above-explained powders are first made available in the form of astarting mixture as explained, and then, with the aid of a press,compressed under increased pressure and brought into the intended shape.Debinding of the green compacts produced in this manner is subsequentlyperformed in a furnace in an inert gas atmosphere, a nitrogen atmospherefor example, or an oxygen-containing gas atmosphere. For this purpose,the compressed starting mixture is heated in the furnace to atemperature of 400° C. to 500° C. and kept there for a period of tenminutes to one hour. The temperature during debinding depends primarilyon the utilized pressing support arrangement, i.e., the micro wax used.To this end, the temperature may also be below the 400° C. mentioned, inthe range of 220° C. to 300° C., for example.

Another heat treatment of the debound, compressed starting mixtureoccurs after debinding in an oxidizing gas atmosphere in a furnace at atemperature of 410° C. to 500° C. The molding is heated in the furnaceto this temperature and is kept there for a period of 20 minutes to 400minutes, 200 minutes, for example. The gas atmosphere in the furnace isair, for example.

This method yields a magnetically soft powder composite material inwhich the utilized soft ferrite powder is at least largely present as agrain boundary phase, i.e., the soft ferrite powder particles enclosethe iron powder particles used in the powder composite material.

The pressing support arrangement used during the course of themanufacturing method facilitates compacting and shaping of the startingmixture during pressing. However, the pressing support arrangementshould be completely removed or evaporated during debinding in such amanner that it does not directly affect the obtainable materialcharacteristic values of the achieved magnetically soft powder compositematerial. This is primarily achieved by using micro wax as the pressingsupport arrangement.

Compacting of the starting mixture in the die under increased pressuremay be performed by uniaxial pressing at a pressure of 500 mPa to 1000mPa.

Finally it should be pointed out that solenoid valves manufactured usingthe magnetically soft powder composite material of the present inventionare absolutely fuel-stable and thermo-stable under typical conditions ofuse in diesel injectors in motor vehicles. In addition, they have a verygood mechanical stress capacity with respect to flexural strength aswell as edge fracture strength.

1. A magnetically soft powder composite material, comprising: at least99.4 wt. % of one of a pure iron powder, a phosphatized iron powder, andan iron alloy powder; and 0.05 wt. % to 0.6 wt. % of a soft ferritepowder.
 2. The magnetically soft powder composite material of claim 1,wherein the soft ferrite powder includes one of an MnZn ferrite powder,an NiZn ferrite powder, and a mixture of the MnZn ferrite powder and theNiZn ferrite powder.
 3. The magnetically soft powder composite materialof claim 1, wherein powder particles of one of the pure iron powder andthe phosphatized iron powder have an average grain size between 30 μmand 150 μm.
 4. The magnetically soft powder composite material of claim1, wherein powder particles of the soft ferrite powder have an averagegrain size of less than 20 μm.
 5. The magnetically soft powder compositematerial of claim 4, wherein the average grain size of the powderparticles is below 5 μm.
 6. The magnetically soft powder compositematerial of claim 5, wherein the average grain size of the powderparticles is below 1 μm.
 7. The magnetically soft powder compositematerial of claim 1, wherein the magnetically soft powder compositematerial has a saturation polarization of more than 1.85 Tesla.
 8. Themagnetically soft powder composite material of claim 7, wherein themagnetically soft powder composite material has a saturationpolarization of 1.90 Tesla to 2.05 Tesla.
 9. The magnetically softpowder composite material of claim 1, wherein the magnetically softpowder composite material has an intrinsic electrical resistance of morethan 1 μΩm.
 10. The magnetically soft powder composite material of claim9, wherein the magnetically soft powder composite material has anintrinsic electrical resistance of 5 μΩm to 15 μΩm.
 11. A method formanufacturing a magnetically soft powder composite material, comprising:a) preparing a starting mixture including a soft ferrite powder and oneof a pure iron powder, a phosphatized iron powder, and an iron alloypowder; b) mixing the starting mixture; c) compacting the startingmixture in a press under increased pressure to form a compacted startingmixture; d) removing a binder from the compacted mixture in one of aninert gas atmosphere and an oxygen-containing gas atmosphere; and e)heat treating the compacted mixture debinded in d) in an oxidizing gasatmosphere at a temperature of 410° C. to 500° C.
 12. The method ofclaim 11, further comprising: adding a pressing support arrangement tothe starting mixture before the mixing step.
 13. The method of claim 12,wherein the pressing support arrangement includes a micro wax.
 14. Themethod of claim 11, wherein the removing is performed at a temperatureof 400° C. to 520° C. over a period of ten minutes to one hour.
 15. Themethod of claim 11, wherein the heat treating is performed over a periodof 20 minutes to 400 minutes.
 16. The method of claim 11, wherein theremoving is performed in one of a nitrogen atmosphere, anoxygen-nitrogen mixture, and air over a period of 10 minutes to 70minutes.
 17. The method of claim 16, wherein the oxygen-nitrogen mixtureincludes 5 vol % to 30 vol % of oxygen.
 18. A rapidly switching solenoidvalve, comprising: a magnetically soft powder composite materialincluding: at least 99.4 wt. % of one of a pure iron powder, aphosphatized iron powder, and an iron alloy powder; and 0.05 wt. % to0.6 wt. % of a soft ferrite powder.
 19. The rapidly switching solenoidvalve of claim 18, wherein the rapidly switching solenoid valve is adiesel injection valve.